Stripping my hull

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rascacio

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Morning all, I've done the obligatory searches but not really got anywhere, so can anyone give me first hand information on stripping my GRP hull ready for painting ?

My 45' boat needs repainting. It's 40 years old and has not been painted for at least 10 years so needless to say the paintwork is worn through to the gelcoat where fenders have rubbed plus other dodgy bits here and there from a lot of travel.

The existing paint is 2 pack (I think) and chalky at this stage and the gelcoat is pretty yellowish so hence the re-paint. I intend to apply the paint (2 pack) by roller and brush, in Greece where the boat is.

So what is the best method to prepare the hull ? Will sodium hydroxide stripper work ? Is sanding better ? Or will a heat gun be of any use ? I realise that this is big job but that's not a problem as I'm allowing about 4 weeks to do it with at least 2 people and possibly 3.

So, any advice from anyone who's been there would be most welcome !!!

Thanks in advance.
 
Sanding is IMO the best and only method.Paint stripper will attack the gelcoat unless you use the weaker type that's suitable for glassfiber and a heat gun will lift it if you apply enough heat to soften the paint.Make sure the old paint is a two pack .If it is you may not need to remove all of it.
 
The yellowing may be exposed epoxy rein used to seal the gel coat, it will much harder to sand than bare gelcoat, so might be checking. If is epoxy you will need an epoxy primer before applying the finish coat.

Sanding is by far the best as it leaves a good key for new coating. A 200 grit will make short work of the old paint if you have an random orbital sander with at least a 3 mm stroke.

To avoid contamination when sanding, you first need to clean the hull, a good wash and dry then wipe down with something like acetone of a suitable thinners, start at the top and work your way down when washing and cleaning with a solvent.

Preparation is 99% of the final outcome and finish.

Good luck and fair winds. :)
 
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Sanding.
Not necessary to remove old paint unless it is evidently failing, that is not holding to gelcoat in substantial portions of the area. Some smaller places where it falls off can be sanded to gelcoat, or just the patches stripped and faired in with putty to surrounds. Sound old paint just needs to be sanded well, coarse sandpaper last, by hand, so to give a grip, and painted over.
But ensure the new paint is compatible with what was there. Or conversion primer may be used, also important to know which. The "2-pack" is not saying what it is, chemically (probably a polyurethane).
 
Thanks to all of you. It is as suspected, elbow grease and sanders at dawn !! To be honest I didn't fancy using stripper anyway even if it was easier. I don't think there is any epoxy below the paint just very old gelcoat, but how would one be able to tell ?
 
Hard to tell. But standard way was to use polyurethane on topsides, right onto gelcoat. No point in using epoxy above water, but was used on bottom, as 'prevention' for osmosis.
Polyurethane may be used on top of both, naturally. Just sand a bit to freshen the surface, take off what's deteriorated.
Polyurethane cannot be put on alkyd (oil-based) paints.
 
rascacio said:
Thanks to all of you. It is as suspected, elbow grease and sanders at dawn !! To be honest I didn't fancy using stripper anyway even if it was easier. I don't think there is any epoxy below the paint just very old gelcoat, but how would one be able to tell ?
Click to expand...

Cured epoxy is almost twice as hard to sand as old gel coat. Simple test. Pick an area that is not very yellow and rub by hand with a block and paper for say 10 rubs. Do the same over a yellow area, if the yellow area has less material removed it may well be epoxy your sanding.

Good luck and fair winds. :)
 
oldsaltoz said:
Cured epoxy is almost twice as hard to sand as old gel coat. Simple test. Pick an area that is not very yellow and rub by hand with a block and paper for say 10 rubs. Do the same over a yellow area, if the yellow area has less material removed it may well be epoxy your sanding.

Good luck and fair winds. :)
Click to expand...

Thanks oldsaltoz. I must clarify and say that my hull beneath the paint is more cream coloured than yellow !! But I will give it your test when I get over there and start prepping.
 
Epoxy primers are used for polyurethane, so there may be such primer under the paint. This because epoxy has very good adhesion. At least you know there is well prepared surface from old gelcoat :)
May be hard kind which will indeed be lot harder than gelcoat, as said above - or a 'high build primer' a bit like putty, made with fillers in so to be sanded smooth and these are made soft, easy to sand. No reason to sand it off as long as sticks.
Btw. Polyurethane is also harder then gelcoat.

Sandpaper - for electric sander the choice depends what equipment and what you want, coarser paper will eat the surface fast, easy to overdo or make holes... Finer may clog with paint. Best idea is to see how it goes. Generally you start with coarse for removing damaged surface, then go gradually to finer.
- I would use coarse one - and caution - for removing failing paint and initial sanding; from 100-200. For a tough one even 40-80 can be used but easy to make deep scratches - so depends how it will be painted later. If thick primer will be used then some scratches are no problem. But hi gloss polyurethane will show every tiny scratch or dent.
- When you have sound surface, paint that sticks, then no reason to spoil it, better to sand smooth from beginning. 200 is about right to sand gelcoat or filler, also a high build primer, but finer can be used. For painted surface 200 may be to coarse.
- When you just want to prepare sound surface for overcoat, that is sand the old paint to put new coat on, or sand over thin primer and between coats - 300-400 grit is enough, it's just to matt the surface. So if you have a good old paint there - don't use less then 300.
But to give better grip on old paint then (after sanding it smooth with finer paper) you can use some 80 grit carefully to just scratch the surface for grip.

Don't forget washing, more important then sanding. Actually sound old paint can be repainted without sanding at all, but must be clean and degreased. Wash hull with strong detergents before sanding, preferably hot water, may be pressure washing. After initial sanding wash again with detergents and follow wiping with solvents. Then on finish sanding wipe again with solvents, no saving on solvents and wipes. But directly before painting don't use water... :)

This is general, when painting follow instructions on the can, every producer states how to prepare surface and coats.
For instance such instruction: http://www.yachtpaint.com/LiteratureCentre/boat-painting-guide-usa-eng.pdf
Much more detailed, any subject: http://www.boatpaint.co.uk/datasheets/International/InternationalDataSheets.html
 
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